1. Field of the Invention
This invention relates to a process for the recovery of fluorine-containing compounds from gypsum precipitates recovered from the acidulation of phosphate rock with sulfuric acid in the presence of potassium ion, and more particularly to a method for the extraction of the fluorides from the gypsum by means of certain phosphoric acid solutions.
2. Description of the Prior Art
Phosphoric acid plants are currently operated utilizing a basic and well known process for the acidulation of phosphate rock which comprises reaction of the rock with sulfuric acid to form phosphoric acid with subsequent reaction of the phosphoric acid with ammonia to produce ammonium phosphates. The phosphoric acid formed in this process is called wet process phosphoric acid. In this reaction, a by-product is gypsum having the chemical formula CaSO.sub.4.2H.sub.2 O. Essentially, all phosphate rock contains some fluorine normally in the 3.0-4.0 percent range and the acidulation reaction usually generates gaseous fluorides which in prior years are usually evolved into the atmosphere or trapped with water scrubbing apparatus.
In recent years, air and water pollution law regulations have become more stringent and are now being enforced vigorously. Therefore, operators of phosphoric acid plants have had many pollution problems with fluorine emission in the atmosphere and with the by-product gypsum from these phosphoric acid plants. An important problem in the operation of these wet process phosphoric acid plants has been in the expensive methods necessary for handling the large amounts of fluorine compounds liberated in the gaseous and aqueous effluents from such plants. In some phosphate complexes from 10,000 to 30,000 tons per year of fluorine compounds may be liberated by various methods and it is estimated that in a typical wet process phosphoric acid plant, a portion of the fluorides are evolved in the atmosphere in gaseous form such as hydrogen fluoride and silicon tetrafluoride which can destroy vegetation and effect other facilities in close proximity to the plant if they are not scrubbed out, and such scrubbing systems are not always effective. A second portion of the fluorine is found in gypsum dumps and is subject to leaching into groundwater and streams. Still another portion of the fluorine remains with the final products and when such final products are used as fertilizers they may introduce fluorine into the soil. Only in recent years have studies been made on the effects of fluorides contained in final products and indications seem clear that they may have a deleterious effect on the long range producing ability of the soil, see for Example Kudzin et al., Chemical Abstracts, 73, 870534 (1970).
There is a great deal of literature and patent art related to attempts to remove the fluoride values from fluorine-containing phosphate rock in the operation of a phosphoric acid plant including methods for suppressing the evolution of fluoride values in the operation of a process and/or attempting to scrub the fluorine from effluent gases and waste water. Two such methods are described in U.S. Pat. No. 2,954,275 and 2,976,141 to Carothers et al. which use sodium or potassium compounds to suppress the fluorides so that they are concentrated in the gypsum cake. These patents indicate that this is achieved by adding a suppressing amount of an alkali metal salt to the acidulation reaction.
Other prior art attempts to overcome the problem of fluorine evolution and a reduction in the amount of fluorine contained in final products and a reference of this type is British Pat. No. 735,086 (1955), which discloses the acidulation of phosphate rock by a two step procedure using a strong mineral acid such as nitric acid or hydrochloric acid. In a similar process in U.S. Pat. No. 3,431,096, a process is disclosed for reducing evolution of fluorine values in formation of triple superphosphate fertilizer by reaction of phosphate rock and phosphoric acid wherein ammonia or urea is added to suppress the fluorine evolution.
Two additional patents of pertinence to processes of this type are U.S. Pat. Nos. 2,567,227 and 2,728,635 to Miller which disclose the acidulation of phosphate rock with phosphoric acid to form monocalcium phosphate, cooling to solidify the monocalcium phosphate and then converting it to dicalcium phosphate by hydrolysis. In the earlier patent, it is indicated that the fluorine in the rock is vaporized in the system, circulates throughout the system and/or leaves the system with the calcium phosphate. The later patent indicates that the process of U.S. Pat. No. 2,567,227 provided a final calcium phosphate product having a fluorine content too high to be of animal feed grade. The solution to this problem in the later patent was the addition of some dilute sulfuric acid in the acidulation step which would of course lead to additional fluorine evolution during the first step.
There are also patents known in the art which indicate that it is known to acidulate phosphate rock with phosphoric acid and to then recover solid monocalcium phosphate by cooling of the resulting solution and recovering the monocalcium phosphate. Processes of this type are disclosed for example in U.S. Pat. Nos. 3,494,735 and 3,645,676. In addition, U.S. Pat. Nos. 3,619,136 and 3,792,151 to Case disclose the reaction of phosphate rock with recycle phosphoric acid at temperatures of about 125.degree.-180.degree. F. (52.degree. C. to 83.degree. C.) to form a solution of monocalcium phosphate, reacting the latter solution with sulfuric acid to produce phosphoric acid and calcium sulfate, precipitating the calcium sulfate, and recycling a portion of the phosphoric acid to the phosphate rock reaction. These patents point out that under the conditions recited, fluorides are not evolved but remain primarily unreacted and may be found with insoluble materials although a portion remains in the phosphoric rock or a solubilized form with stoichiometric amounts of sulfuric acid and KHSO.sub.4 in combination with other steps, and this reaction is described in U.S. Pat. Nos. 3,697,246 and 3,718,453.
In copending application Ser. No. 608,973, filed Aug. 29, 1975, of one of us, there is disclosed a process for the acidulation of phosphate rock and production of substantially pure alkali metal phosphates, calcium phosphates and phosphoric acid which comprises primarily the steps of acidulating phosphate rock with a phosphoric acid solution containing sufficient alkali metal values to provide potassium ions in the system and thus form an insoluble precipitate comprising a mixture of impurities, silicas and fluorides from which the fluorides can be recovered in usable form. It is a feature of this disclosure that the fluorides are not evolved into the atmosphere but are primarily recovered in the insoluble precipitate removed prior to gypsum precipitation. In addition, in prior U.S. Pat. No. 3,840,639 there is disclosed a process for the acidulation of phosphate rock by reaction of the rock with phosphoric acid in the presence of potassium ion.
There are many disclosures of processes in the art concerned with the acidulation of phosphate rock with sulfuric acid and other acids to form usually phosphoric acid and a gypsum precipitate. In such processes, the amount of fluorides evolved in the system have been a continuing problem as they are usually evolved into the atmosphere. In our copending prior filed application Ser. No. 696,290, filed June 15, 1976 there is disclosed a process whereby phosphate rock is acidulated with sulfuric acid in the presence of a controlled amount of potassium ion as K.sub.2 O and a controlled amount of SiO.sub.2, by which process the evolution of the fluorides is suppressed and precipitates with the gypsum precipitate. In this procedure, the resulting mixture of phosphoric acid and the precipitated gypsum is then separated to provide a phosphoric acid solution and a gypsum precipitate, the latter being substantially contaminated with the precipitated fluorides. The present invention is an improvement on this process.
In prior procedures, the art has not considered that it was necessary to recover the fluorides from the gypsum cake as the gypsum was usually passed to a gypsum pond. However, in view of the recent environmental considerations and because the present day economics, it is desirable that the gypsum be purified for use in other areas and also desirable to recover the fluorides contained in the gypsum.
In prior art relating to treatment of the gypsum, U.S. Pat. No. 3,795,728 to Scheel discloses a method for recovering alkali metal silicofluorides from gypsum by washing or leaching the gypsum with a hot wash filtrate, i.e., hot water. Usually, the wash filtrate has been cold saturated with alkali silicofluoride and the alkali silicofluoride contained in the extract is separated from the gypsum by extraction and crystallization.
U.S. Pat. No. 3,420,622 to Donges et al. disclose a method for recovering fluorine compounds by digesting phosphates with nitric acid, forming a suspension of finely divided calcium silicofluoride, extracting this mixture with acidified water and precipitating the silicofluoride in the form of potassium silicofluoride. In U.S. Pat. No. 3,579,322 to Higgins there is disclosed a concept of recycling a stream of pond water to a phosphate water plant operation which stream of pond water contains phosphates, sulfates, and silicofluorides and the silicofluorides are extracted by means of a cation exchange system.
U.S. Pat. No. 3,915,659 to Kadotani et al., discloses the purification of sodium silicofluoride by repulping sodium silicofluoride containing gypsum and/or phosphates. The repulping procedure effects substantial removal of impurities and obtains sodium silicofluoride free from gypsum with the process comprising the addition of an alkali metal material thereto.
Further, a large number of patents are known in the prior art which are concerned with the precipitation and recovery of alkali metal silicofluorides from phosphoric acid. However, the problems in recovery of the silicofluoride from the acid are distinctly different from those presented with respect to recovery of the alkali silicofluorides from gypsum since the patents indicate that the fluorides my be simply precipitated from the phosphoric acid solution.
The present invention provides a procedure whereby the acidulation reaction may be conducted under conditions such that the fluorides are precipitated with the gypsum leaving a phosphoric acid solution substantially free of fluorides and thereafter these fluorides are effectively extracted from the gypsum mixture in recoverable form.